Solves the "filter opened" problem on the list

this pr should solve the problems of filters opened in the gallery and media lists, [see here](https://github.com/sonata-project/SonataMediaBundle/issues/108)

**CAREFUL!!!**
[this pull request](https://github.com/sonata-project/SonataAdminBundle/pull/623) on SonataAdminBundle should be merged first, bacause the twig template uses the ->isHidden() method on the filter, and ->hasActiveFilters() method with the $excludeHidden parameter to check for active filters.